Electrodynamometer

ABSTRACT

An electrodynamometer for measuring the torque of a motor. The electrodynamometer includes a supporting structure and a stator mounted for rotation on the structure and having a winding adapted for energization by a variable source of direct current to produce a predetermined number of magnetic poles. A squirrelcage rotor is mounted for rotation within the stator and coupled to the motor the torque of which is to be measured. The rotation of the rotor of the electrodynamometer develops a torque tending to rotate the stator. A spring is connected between the stator and the structure for absorbing the torque exerted on the stator by the rotor and the angle of rotation of the stator against the action of the spring is measured on a scale for providing an indication of the torque of the motor.

United States Patent Wildi 14 1 Apr. 25, 1972 54] ELECTRODYNAMOMETER3,029,634 4/1962 Schmitz ..73/134 [72] Inventor: Theodore Wildi, Quebec,Canada Primary Examiner charles ARueh] [73] Assignee: Lab-Volt (Quebec)Limited, Quebec, Attorney-Raymond A. Robic Province of Quebec, Canada 22Filed: July 6,1970 [57] ABSTRACT Appl. No.: 52,177

An electrodynamometer for measuring the torque of a motor. Theelectrodynai'nometer includes a supporting structure and a statormounted for rotation on the structure and having a winding adapted forenergization by a variable source of direct current to produce apredetermined number of magnetic poles. A squirrel-cage rotor is mountedfor rotation within the stator and coupled to the motor the torque ofwhich is to be measured. The rotation of the rotor of theelectrodynamometer develops a torque tending to rotate the stator, Aspring is connected between the stator and the structure for absorbingthe torque exerted on the stator by the rotor and the angle of rotationof the stator against the action of the spring is measured on a scalefor providing an indication of the torque of the motor.

9 Claims, 9 Drawing Figures MTENTEDAPR 25 I972 3. 657, 923 SHEET 1 UF 3PRIOR ART PRIOR ART nebaare W. D/

PmpR 'ART mm Arm/m0 PKTENTEDAPMS I972 657 92 3 SHEET 2 OF 3 FIG. 4-

PRIOR ART SPEED TORQUE SPEED Go g F756 l l T' //VV/WOR 23 max ffiOl/Of?WM 0/ PATENTED APR 2 5- I972 SHEET 3 BF R m/ v, #0 M an R M m w A w .0 bg A f A pm 9 FIGS - ELECTRODYNAMOMETER This invention relates to anelectro-meehanical apparatus commonly known as an electrodynamometer formeasuring the mechanical torque developed by a motor or other device.

One type of electrodynamometer which is used in industrial andlaboratory applications consists of a copper disc mounted for rotationand coupled to amotor the torque ofrwhich is to be measured. .Apermanent magnet which may be placed more or less close to the copperdisc produces a flux causing a voltage to be induced in the disc whenthe latter is turned. The A induced voltage gives rise. to aninducedcurrent, the magnitude of which depends principally upon theelectrical resistance offered by the disc. Known theoreticalconsiderations show thatthe. torque developed depends directly upon thespeed of rotation and upon the square of the effective flux which goesacross the disc from the permanent magnet. Measurement of this torquemaybe done by attaching a spring to the permanent magnet and by providingsuch spring with a pointer adapted to move across a scale. Thespeed-torque relationship of such a dynamometer, for a given' setting ofthe permanent magnet, increases linearly with the speed and the torqueis very low when the speed is low.

Another commonly known type of electrodynamometer is similar to the oneabove in that it also uses a copper disc. However, the permanent magnetis replaced by an electromagnet which is fixed with respect to the discbut is energized by a source of variable current, thereby .changing theflux which crosses the copper disc. In this set up, the mechanicaldisplacement of the magnet .is no longer necessary because the flux canbe made to vary according to the current rather than according to amechanical displacement. Again, in this case, the torque is directlyproportional to the speed and is very low when the speed is low. t

Most electric motors develop quite powerful starting torques which mayrange up to 300 percent of their nominal full load value. However, theknown dynamometers produce a high torque only at high speeds.Consequently, the known dynamometers are not entirely satisfactorybecause they do not have the inherent property to measure high torquesat low speed.

It isftherefore the main feature of the invention to provide anelectrodynamometer having speed-torque characteristics which are moredesirable for electric motors developing high starting torques. 1 v

The electrodynamometer, in accordance with the invention, includes asupporting structure and a stator mounted for rotation on the supportingstructure and having winding adapted for energization by a variablesource of direct current to produce a predetermined number of magneticpoles. A squirrel-cage rotor is mounted for rotation within the statorand coupled to the motor the torque of which is to be measured. Therotation of the rotor of the electrodynamometer develops a torquetending to rotate the stator. A spring is connected between the statorand the structure for absorbing the torque exerted on the stator by therotor. Means are provided for measuring the amount of rotation of thestator against the action of the spring for providing an indication ofthe torque of the motor.

The spring interconnecting the stator and the structure supporting thestatormay advantageously be a torsional spring mounted coaxially withthe stator. One end of the stator may be provided with a scale and apointer may be located on the supporting structure for indicating theangle of rotation of the stator. The scale is calibrated for indicatingdirectly the torque of the motor.

The invention will now be disclosed with reference to the accompanyingdrawings illustrating a preferred embodiment of the invention and inwhich:

- FIG. 1 illustrates a known dynamometer used for measuring the torqueof a motor;

FIG. 2 illustrates the speed-torque characteristics of such a knowndynamometer; I

FIG. 3 illustrates another type of known dynamometer;

FIG. 4 illustrates the speed-torque characteristics of an electricmotor; I

FIG. 5 is a diagrammatic view of an electrodynamometer inaccordance'with the invention;

FIG. 6, illustrates the speed-torque characteristics of theelectrodynamometer'in accordance with the invention as well as that ofFIG. 2 for comparison purposes;

I FIG. 7 illustrates the outside structure of the electrodynamometer ofFIG. 5;

- The permanent magnet produces a flux 4: which causes a voltage to beinduced in the disc when the latter turns. The value of this voltagedepends on both the flux and the speed of rotation (a of the disc. Thisinduced voltage produces an induced current whose magnitude depends uponthe electrical resistance R offered by the disc. Known theoreticalconsiderations show that, at moderate speeds, the torque developed isapproximately proportional to dim/R, and depends therefore directly uponthe speed of rotation and upon the square of the effective flux whichcrosses the disc from the permanent magnet. This speed-torquerelationship is shown in FIG. 2 (and again as curve B in FIG. 6) inwhich it can be seen that, for a given setting of permanent magnet 18 (d=1, =O.7), the torque increases linearly with the speed. It is alsoclear that the torque is zero when the disc does not rotate.

Measurement of this torque can be made by attaching a spring 24 to thepermanent magnet and a pointer 26 to the spring. The pointer is free tomove across a scale 28 when the permanent magnet is moved around axis 29of the supporting member 22 by the rotation of the disc.

Another type of electrodynamometer which is commonly used employs anelectromagnet 30 as shownin FIG. 3. This magnet is fixed as to positionand receives a current (i) from a source 32. The value of this directcurrent can be varied by appropriate means thereby changing the fluxwhich crosses the copper disc 10. The set-up is essentially similar tothe one shown in FIG. 1, except that mechanical displacement of themagnet is no longer necessary because the flux can be made to varyaccording to a current rather than according to a mechanical movement.Again, in this case, the torque is directly proportional to speed and isvery low when the speed is low.

Most electric motors develop quite powerful starting torques which mayrange up to, and even exceed, 300 percent of their normal full loadvalue such as. illustrated in FIG. 4. Ideally, inorder to measure thesetorques, a dynamometer should have characteristicssuch that at lowspeed, its torque is large while at high speeds, its torque is somewhatlower.

Clearly, the dynamometers described in FIGS. 1 and 2 do not result. Theyinherently give a high torque at high speed and low torque at low speed.

In order to improve the torque-speed characteristics of the dynamometer,the arrangement shown diagrammatically in FIG. 5 was conceived. Itcomprises a conventional stator 34 such as used in commercial inductionmotors and carrying a winding 36 distributed in the slots 38 thereof soas to obtain two or more poles. The winding 36 is brought out to twoterminals 40 and 42 which will be excited by a variable direct currentso as to vary the magnetic flux within the dynamometer.

The stator is free to rotate on roller supports 44 and anyefinterconnectingthe stator and a fixed support. A standardsquirrel-cage die-cast rotor 48 having end rings 50 and bars 52 isrotatably mounted within the stator 34.

When the rotor is made to turn by some external agency, the rotor bars52 cut across the flux created by the stator winding 36 thereby inducinga voltage and a current in the squirrel-cage of the rotor 48. The valueof the voltage depends upon both the speed and the flux of winding 36.The current, however, depends not only upon the resistance of the rotor,but also upon its inductance, and therein lies the major difference ofthe dynamometer in accordance with the invention with the previouslydescribed dynamometers. Indeed, as will be shown, the inductance has avery important effect upon the speed-torque characteristics of thismachine.

The speed torque curve of this new dynamometer is shown as curve A inFIG. 6 and it can be seen that high torques are developed at low speedsand moderate torques at' high speeds. For purposes of comparison, curveB shows the torque-speed curve of the dynamometers discussed in FIGS. 1and 3. From curves A and B, it is clear that the new dynamometer is moresuited to measuring the starting and running torque of motors. Thetorque attains a maximum value T at a speed (o The value of this maximumtorque as well as the speed at which it occurs depends upon theelectrical design of the machine.

A theoretical analysis of this dynamometer shows that for a two-poleconfiguration the torque T is given by the equation.

T= K a) M F R/(R w L in which T= torque in ft.lbs

w speed of rotation in radians/sec.

M mutual inductance between the stator and rotor windings (in henrys) IDC current in the stator winding (in amperes) R Resistance of the rotorwinding (in ohms) L Self-inductance of the rotor (in henrys) K Aconstant which depends upon the units employed above.

Torque-speed curve A of FIG. 6 is a direct result and a graphicalrepresentation of this equation. From this equation we can also deducethat the maximum torque T is developed at a speed m, where:

Dynamometers which'have to develop large torques at low speeds, musttherefore have rotors in which the ratio R/L is low. This implies rotorshaving a larger inductance and low resistance. Squirrel-cage rotors havethis basic property. On the other hand, the copper disc rotors describedin FIGS. 1 and 2 possess very low inductance, with the result that theirmaximum torque is developed only at very high speeds. This is why copperdisc dynamometers are not suited to measuring the starting torques ofcommercial induction rotors.

The speed-torque curve of this dynamometer is better adapted to measurethe torque-speed characteristics of induction motors as well as DCmotors. It should be noted that it also develops no torque when thespeed is zero. However, the dynamometer can be designed to develop verypowerful torques at speeds as small as percent of full load speed, whichis sufficiently close to the stalling point of the motor under test.Indeed, it is often preferable to have the machine crawling at a lowspeed, rather than completely stalled, because this tends to show uptorque variations due to cogging.

Returning to FIG. 5, owing to the torque developed between the rotor 48and the stator 34, the stator tends to revolve in the same direction asthe rotor and is only retained in place by spring 46. The torquedeveloped by the rotor is completely transmitted to the stator, exceptfor minor frictional losses and consequently, a pointer 54 attached tospring 46 may be used to measure the torque developed according to thegraduation across a scale 56.

At this point, it should be mentioned that a linear spring such asspring 46 in the diagram would have to be quite long to get any accuratereading on scale 56. This is rather difiicult to obtain in reasonablespace and for this reason, in practice, the

linear spring 46 is replaced by a torsional spring coaxial with stator34 and permitting afar greater rotation of the stator while stillretaining complete linearity.

FIG. 7 shows the essential external structure parts of the newdynamometer. The stator 34 is supported on trunion bearings 60 which inturn are attached to supports 62'and 64 at the rear and the front end ofthe dynamometer respectively by clips 65. A pulley 66 is connected tothe motor under test for which the torque-speed characteristics isdesired. The stator 34 is directly connected to a torsional spring 68 byclamping member 70.Spring 68 is in turn clamped to a shaft 72 by meansof clamping member 74. Shaft 72 is rotatably mounted in support 76which, in turn, is secured to extension member 78 attached to support62.

The stator winding is brought out through the center of the spiralspring 68 by means of wires 80.

A scale 82 is located at the pulley end of the stator and is shown inmore detail in FIG. 8. In effect, a 270 scale is used. The calibrationof the scale can be varied by changing the number of spirals between thestator 34 and support 76, and can be made permanent by tighteningclamping members 70 and 74 in position after calibration. The torquedeveloped by the dynamometer is then varied by changing the excitationof the stator winding and by noting the deflection of the scale 82against a pointer 84.

Stops 85 are secured to the stator to prevent more than one revolutionof the stator.

The direct current for the stator winding is conveniently obtained bythe circuit shown in FIG. 9. In this circuit, an alternating voltage isapplied to tenninals 86 and 88 to the input of an auto-transformer 90connected in series with such ter- 7 minals and having a variable tap92. A voltage limiting resistor 93 is connected between tap 92 and oneterminal of auto transformer 90. The output from the transformer tap 92is fed to a diode bridge 94 so as to obtain a DC output which is appliedto the terminals of the stator winding. No filtering is required becauseof the high inductance of the winding which in itself, acts as asatisfactory filter and assures a steady DC current in the stator.

The advantages of the above described dynamometer are as follows:

I. The torsional spring gives a very'large scale deflection withoutsacrificing linearity and without having to 'use mechanical multipliers,such as rack and pinion systems. A linear spring could be used but wouldhave required a much larger space to obtain a satisfactory scaledeflection without the use of mechanical multipliers.

2. The inherent torque-speed properties of this dynamometer are muchmore adapted to the requirements of commercial motors. The torque ishigh when the speed is low and moderate when the speed is high.

3. The large rotor surface permits good heat dissipation which issomewhat restricted in disc-type dynamometers. In addition, the radialfans which are usually part of the die-casting on the rotor providesatisfactory cooling means for the machine.

I claim:

1. An electrodynamometer for measuring the torque of a motor comprising:

a. a supporting structure;

b. a stator mounted for rotation on said structure and including awinding adapted for energization by a variable source of direct currentto produce a predetermined number of magnetic poles;

. a squirrel-cage rotor mounted for rotation within said stator andcoupled to said motor for developing between said rotor and said statora torque tending to rotate and stator;

d. a torsional spring mounted coaxially with the axis of rotation of therotor and having a predetermined number of spirals, said torsionalspring being secured at one end to one end of said stator and at theother end to said supporting structure and being adapted to absorb thetorque exerted on said stator by said rotor; and

e. means for measuring the amount of rotation of said stator against theaction of said spring for providing an indica- 'tion of the torqueofsaid motor.

2. An electrodynamometer as defined in claim 1, wherein saidsquirrel-cage rotor has a low ratio of resistance to inductance.

3. An electrodynamometer as defined in claim 1, wherein said supportingstructure includes two upstanding portions located one at each end ofthe stator, each upstanding portion holding a trunion bearing rotatablysupporting said stator.

4. An electrodynamometer as defined in claim 1, further comprising ashaft for said squirrel-cage rotor, and a pulley secured to said shaftand adapted for connection to said motor. a

5. An electrodynamometer as defined in claim 1 further comprisingclamping members for securing said spring at each end thereof to saidstator and to said supporting structure respectively, said clampingmembers tightened in position after calibration of theelectrodynamometer.

6. An electrodynamometer as defined in claim 1, further comprising ascale secured to one end of said stator and a pointer located on saidsupporting structure, said scale being calibrated so as to indicatedirectly the torque of the motor.

7. An electrodynamometer as defined. in claim 6, wherein said statorincludes stop means for preventing more than one revolution of thestator.

8. An electrodynamometer as defined in claim 1, wherein said directcurrent source is connected to said winding through conductors passingthrough the center of said torsional spring.

9. An electrodynamometer as defined in claim 1, wherein said variablesource of direct current comprises an alternating current source havinga first and a second terminals, a variable autotransformer having oneterminal connected to the first terminal of said alternating currentsource and an intermediate movable tap, a voltage-limiting resistorconnected between said tap and the other terminal of saidautotransformer, and a diode bridge connected between the movable tapand the second terminal of said alternating current source forrectifying said alternating current, thus providing a variable source ofdirect current, for the stator winding.

1. An electrodynamometer for measuring the torque of a motor comprising:a. a supporting structure; b. a stator mounted for rotation on saidstructure and including a winding adapted for energization by a variablesource of direct current to produce a predetermined number of magneticpoles; c. a squirrel-cage rotor mounted for rotation within said statorand coupled to said motor for developing between said rotor and saidstator a torque tending to rotate and stator; d. a torsional springmounted coaxially with the axis of rotation of the rotor and having apredetermined number of spirals, said torsional spring being secured atone end to one end of said stator and at the other end to saidsupporting structure and being adapted to absorb the torque exerted onsaid stator by said rotor; and e. means for measuring the amount ofrotation of said stator against the action of said spring for providingan indication of the torque of said motor.
 2. An electrodynamometer asdefined in claim 1, wherein said squirrel-cage rotor has a low ratio ofresistance to inductance.
 3. An electrodynamometer as defined in claim1, wherein said supporting structure includes two upstanding portionslocated one at each end of the stator, each upstanding portion holding atrunion bearing rotatably supporting said stator.
 4. Anelectrodynamometer as defined in claim 1, further comprising a shaft forsaid squirrel-cage rotor, and a pulley secured to said shaft and adaptedfor connection to said motor.
 5. An electrodynamometer as defined inclaim 1 further comprising clamping members for securing said spring ateach end thereof to said stator and to said supporting structurerespectively, said clamping members tightened in position aftercalibration of the electrodynamometer.
 6. An electrodynamometer asdefined in claim 1, further comprising a scale secured to one end ofsaid stator and a pointer located on said supporting structure, saidscale being calibrated so as to indicate directly the torque of themotor.
 7. An electrodynamometer as defined in claim 6, wherein saidstator includes stop means for preventing more than one revolution ofthe stator.
 8. An electrodynamometer as defined in claim 1, wherein saiddirect current source is connected to said winding through conductorspassing through the center of said torsional spring.
 9. Anelectrodynamometer as defined in claim 1, wherein said variable sourceof direct current comprises an alternating current source having a firstand a second terminals, a variable autotransformer having one terminalconnected to the first terminal of said alternating current source andan intermediate movable tap, a voltage-limiting resistor connectedbetween said tap and the other terminal of said autotransformer, and adiode bridge connected between the movable tap and the second terminalof said alternating current source for rectifying said alternatingcurrent, thus providing a variable source of direct current, for thestator winding.